Modular Multilevel Converters (MMC) may be used as Static VAR Compensators (Statcoms) and/or Flexible AC Transmission Systems (FACTS) connected to an electrical grid supplying electrical power to a load. Specifically, a Statcom is connected at a Point of Common Coupling (PCC) to an electric power grid that is negatively affected by current harmonics and/or reactive power injected by a load connected to the grid. The task of the Statcom is then to provide active power filtering, by injecting harmonics and/or reactive power into the PCC, such that the grid currents are free of harmonics and are in phase with the grid voltage, resulting in zero reactive power.
Conventional control of modular multilevel converters in general and Statcoms in particular includes two hierarchical control layers. The upper control layer is based on vector control with a modulator. Multiple PI loops may be used to control the load and the circulating currents by manipulating the voltage reference of a modulator. Typically, carrier-based Pulse Width Modulation (PWM) or Space Vector Modulation (SVM) may be used as modulator. The lower control layer utilizes a redundancy in converter states (i.e. groups of switching states that produce the same line to line voltage) in order to balance the capacitor voltages.
The patent application WO2014/033155 relates to model predictive control of a DC/AC converter supplying electrical power to a load such as a motor, with load currents being exclusively determined by the branch currents of the converter. A first (MPC) control stage supersedes the vector control of the upper control layer as described above, and manipulates a real-valued voltage reference to be provided to a second (PWM modulator) control stage.
The patent application WO2014/006200 discloses a Model Predictive Direct Current Control MPDCC scheme for a Modular Multilevel Converter in Statcom applications. Optimum converter module switching sequences with a trajectory of the controlled variables (currents, voltages) respecting predefined hysteresis bounds are determined. The MPC scheme directly manipulates the individual switch positions or switching states of the power semiconductor switches in the converter modules. An intermediate control stage such as a modulator is not required, nor are there any sorting lists being used in a balancing stage. MPDCC is reported to achieve a very fast current response during power-up and power-down transients as compared to a PWM based scheme.
Active power filtering capabilities of an MMC Statcom include one or more of                Injection of appropriate harmonics into the PCC to compensate for load harmonics.        Compensation of reactive power at the PCC.        Injection of negative sequence currents to compensate for imbalance in the load.        Very fast dynamic response to load transients from arc furnaces, faults in the grid or the load, or low-voltage ride through operations.        